After using both the BB for a few years, a WM5 device (Blackjack) and 4 Treos, the main thing in my opinion is the stability of the OS. The BB doesn't reboot itself on a random basis for no reason at all, nor does it need rebooting on a frequent basis as does many WM devices.
However, the push email was the main reason I switched. Email is automatically sent to my BB without me having to do anything other than the initial set up. It works, it isn't broken, so there is no reason to fix it.
Security is another big plus, as is security, build quality and the PDA features (email, contacts, calendar and phone) in one device.